1. Field of the Invention
The present invention relates to the fabrication of integrated circuits. More particularly, the invention relates to a process and apparatus for depositing dielectric layers on a substrate.
2. Background of the Invention
One of the primary steps in the fabrication of modern semiconductor devices is the formation of metal and dielectric films on a substrate by chemical reaction of gases. Such deposition processes are referred to as chemical vapor deposition or CVD. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. The high temperatures at which some thermal CVD processes operate can damage device structures having layers previously formed on the substrate. A preferred method of depositing metal and dielectric films at relatively low temperatures is plasma-enhanced CVD (PECVD) techniques such as described in U.S. Pat. No. 5,362,526, entitled xe2x80x9cPlasma-Enhanced CVD Process Using TEOS for Depositing Silicon Oxidexe2x80x9d, which is incorporated by reference herein. Plasma-enhanced CVD techniques promote excitation and/or disassociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma of highly reactive species. The high reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such PECVD processes.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore""s Law), which means that the number of devices that will fit on a chip doubles every two years. Today""s fabrication plants are routinely producing devices having 0.35 cm and even 0.18 xcexcm feature sizes, and future plants will be producing devices having even smaller geometries. Jeng et al. in xe2x80x9cA Planarized Multilevel Interconnect Scheme with Embedded Low-Dielectric-Constant Polymers for Sub-Quarter-Micron Applicationsxe2x80x9d, published in the Journal of Vacuum and Technology in June 1995, describes the use of a low dielectric constant polymeric material, such as parylene, as a substitute for silicon dioxide (SiO2) between tightly spaced conductive lines or other strategically important areas of an integrated circuit structure. Parylene, a generic name for thermoplastic polymers and copolymers based on p-xylylene and substituted p-xylylene monomers, has been shown to possess suitable physical, chemical, electrical, and thermal properties for use in integrated circuits. Deposition of such polymers by vaporization and decomposition of a stable cyclic dimer, followed by deposition and polymerization of the resulting reactive monomer, is discussed by Ashok K. Sharma in xe2x80x9cParylene-C at Subambient Temperaturesxe2x80x9d, published in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26; at pages 2953-2971 (1988). Properties of such polymeric materials, including their low dielectric constants, are further discussed by R. Olson in xe2x80x9cXylylene Polymersxe2x80x9d, published in the Encyclopedia of Polymer Science and Engineering, Volume 17, Second Edition, at pages 990-1024 (1989).
Several parylene films have been developed for deposition within integrated circuits. Parylene-N is deposited from unsubstituted p-xylylene at substrate temperatures below about 70-90xc2x0 C. The parylene-N films typically do not adhere well to silicon oxide and other semiconductor surfaces. Furthermore, parylene-N films typically have poor thermal stability at temperatures above about 400xc2x0 C. Thermal stability of parylene films is improved by fluorinating or chlorinating the cyclic dimer of p-xylylene to make parylene-F films or parylene-C films. However, the substituted p-xylylene cyclic dimers are even more expensive than the unsubstituted cyclic dimer and are more difficult to process. Copolymers of p-xylylene and fluorinated or chlorinated monomers may also improve thermal stability. However, the fluorine or chlorine within the films can corrode metal electrical interconnects when an electrical bias is applied.
Parylene films can be deposited by thermal deposition methods or plasma assisted deposition methods. The mechanical properties of the deposited parylene films have not been improved by plasma assistance, and parylene films have remained inferior to other dielectric films for producing integrated circuits. Copolymer films produced from parylene-N and a comonomer have been investigated to improve mechanical properties of the deposited films. However, few copolymer films have been shown to contain sufficient amounts of the comonomer to influence the mechanical properties of the deposited film.
There remains a need for parylene films having low dielectric constants and good mechanical properties for use in sub-micron semiconductor devices.
The present invention provides a method and apparatus for depositing a low k dielectric layer from a source of p-xylylene and a comonomer having carbon-carbon double bonds. In particular, a method and apparatus is provided for plasma assisted production of parylene copolymer films having low dielectric constants and improved mechanical properties in comparison to parylene films. A suitable apparatus and method provides for plasma-energized formation of reactive p-xylylene and a reactive comonomer using from about 0 to about 100 W of constant high frequency RF power, or from about 20 to about 160 W of pulsed high frequency RF power. A parylene copolymer is then deposited on a substrate wherein the copolymer contains at least 1% by weight of the comonomer. The comonomers comprise one or more carbon-carbon double bonds. The multivinyl compounds may further comprise silicon-carbon bonds and silicon-oxygen bonds. Preferred comonomers comprise two or more carbon-carbon double bonds and include tetravinyltetramethylcyclotetrasiloxane, tetraallyloxysilane, and trivinylmethylsilane. The comonomer is preferably combined with the p-xylylene in an amount from about 1% by weight to about 15% by weight. Plasma assistance increases the deposition rate of the copolymer layer and increases the amount of comonomer that combines with the p-xylylene.